I think it's just to do with you have done your bit and it makes you feel more at ease should we lose! I am like you have had lucky tops, boxer shorts, socks and any combination of the above!
If I stumble on something when we win games we are underdogs I keep doing it i.e. not shaving until we lose or whatever! It's total madness when you think about it!
However I justify it all by thinking that there must be thousands like me and if we lose after I have done all my things it must be someone in those thousands that forgot!
I am off for a lie down in my padded room.